The molecules listed below were detected by spectroscopy. Their spectral features are generated by transitions of component electrons between different energy levels, or by rotational or vibrational spectra. Detection usually occurs in radio, microwave, or infrared portions of the spectrum.[1]

Interstellar molecules are formed by chemical reactions within very sparse interstellar or circumstellar clouds of dust and gas. Usually this occurs when a molecule becomes ionized, often as the result of an interaction with a cosmic ray. This positively charged molecule then draws in a nearby reactant by electrostatic attraction of the neutral molecule's electrons. Molecules can also be generated by reactions between neutral atoms and molecules, although this process is generally slower.[2] The dust plays a critical role of shielding the molecules from the ionizing effect of ultraviolet radiation emitted by stars.[3]

The first carbon-containing molecule detected in the interstellar medium was the methylidyne radical (CH) in 1937.[6] From the early 1970s it was becoming evident that interstellar dust consisted of a large component of more complex organic molecules (COMs),[7] probably polymers. Chandra Wickramasinghe proposed the existence of polymeric composition based on the molecule formaldehyde (H2CO).[8]Fred Hoyle and Chandra Wickramasinghe later proposed the identification of bicyclic aromatic compounds from an analysis of the ultraviolet extinction absorption at 2175A.,[9] thus demonstrating the existence of polycyclic aromatic hydrocarbon molecules in space.

In 2004, scientists reported[10] detecting the spectral signatures of anthracene and pyrene in the ultraviolet light emitted by the Red Rectangle nebula (no other such complex molecules had ever been found before in outer space). This discovery was considered a confirmation of a hypothesis that as nebulae of the same type as the Red Rectangle approach the ends of their lives, convection currents cause carbon and hydrogen in the nebulae's core to get caught in stellar winds, and radiate outward.[11] As they cool, the atoms supposedly bond to each other in various ways and eventually form particles of a million or more atoms. The scientists inferred[10] that since they discovered polycyclic aromatic hydrocarbons (PAHs) — which may have been vital in the formation of early life on Earth — in a nebula, by necessity they must originate in nebulae.[11]

In 2010, fullerenes (or "buckyballs") were detected in nebulae.[12] Fullerenes have been implicated in the origin of life; according to astronomer Letizia Stanghellini, "It's possible that buckyballs from outer space provided seeds for life on Earth."[13]

In October 2011, scientists found using spectroscopy that cosmic dust contains complex organic compounds ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.[14][15][16] The compounds are so complex that their chemical structures resemble the makeup of coal and petroleum; such chemical complexity was previously thought to arise only from living organisms.[14] These observations suggest that organic compounds introduced on Earth by interstellar dust particles could serve as basic ingredients for life due to their surface-catalytic activities.[17][18] One of the scientists suggested that these compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."[14]

In August 2012, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellarbinaryIRAS 16293-2422, which is located 400 light years from Earth.[19][20] Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.[21]

In 2013, Dwayne Heard at the University of Leeds suggested[26] that quantum mechanical tunneling could explain a reaction his group observed taking place, at a significantly higher than expected rate, between cold (around 63 Kelvin) hydroxyl and methanol molecules, apparently bypassing intramolecular energy barriers which would have to be overcome by thermal energy or ionization events for the same rate to exist at warmer temperatures. The proposed tunneling mechanism may help explain the common observation of fairly complex molecules (up to tens of atoms) in interstellar space.

A particularly large and rich region for detecting interstellar molecules is Sagittarius B2 (Sgr B2). This giant molecular cloud lies near the center of the Milky Way galaxy and is a frequent target for new searches. About half of the molecules listed below were first found near Sgr B2, and nearly every other molecule has since been detected in this feature.[27] A rich source of investigation for circumstellar molecules is the relatively nearby star CW Leonis (IRC +10216), where about 50 compounds have been identified.[28]

The following tables list molecules that have been detected in the interstellar medium, grouped by the number of component atoms. If there is no entry in the Molecule column, only the ionized form has been detected. For molecules where no designation was given in the scientific literature, that field is left empty. Mass is given in Atomic mass units. The total number of unique species, including distinct ionization states, is listed in parentheses in each section header.

Most of the molecules detected so far are organic. Only one inorganic species has been observed in molecules which contain at least five atoms, SiH4.[30] Larger molecules have so far all had at least one carbon atom, with no N-N or O-O bonds.[30]

Evidence for the existence of the following molecules has been reported in scientific literature, but the detections are either described as tentative by the authors, or have been challenged by other researchers. They await independent confirmation.

^Iglesias-Groth, S. et al. (2008-09-20), "Evidence for the Naphthalene Cation in a Region of the Interstellar Medium with Anomalous Microwave Emission", The Astrophysical Journal Letters685: L55–L58, arXiv:0809.0778, Bibcode:2008ApJ...685L..55I, doi:10.1086/592349 - This spectral assignment has not been independently confirmed, and is described by the authors as "tentative" (page L58).